Method of making a pressure bonded composite member

ABSTRACT

The application describes a composite article suitable for fabrication into an automotive trim member and a process for making it. The composite article comprises stainless steel bonded to a non-ferrous metal that is electrochemically anodic to carbon steel. Formation of the bond which is preferably mechanical is dependent upon heat and pressure. The bond is preferably produced by using non-ferrous metal in wire form and without any mechanical or chemical cleaning of the stainless steel and nonferrous metal for removal of surface oxides.

United States Patent Zaremski et al. 451 A g, 22, 1972 [54] METHOD OFMAKING A PRESSURE 3,292,256 12/1966 Morgan ..29/501 X BONDED COMPOSITEMEMBER 3,340,597 9/ 1967 Stein et a1 ..29/498 X [72] Inventors, DonaldZaremski, cheswick; 3,481,023- 12/1969 .lost et a] ..29/498 X 2,050,0468/1936 Everlmg ..73/236 Jack William 2 706850 4/1955 5 'oumet m1 72/236x D. Heavner, Jr., Lower Burrell, all e] of OTHER runucmrows Assigneei fy Ludlum Steel p The Structure and Properties of Materials" byPittsburgh Brophy et 3.1. pages 166- 175. Volume 11.

[22] Filed: April 29, 1969 Primary Examiner-John F. Campbell PP N04830,179 Assistant Examiner-Richard Bernard Lazarus Related u.s.Application Data f gg gflgggg g 2 3:? and [63] Continuation-in-part ofSer. No. 706,495, Feb.

19, 1968, abandoned. [57] ABSTRACT The application describes a compositearticle suitable 323: for fabrication into an automotive trim member anda process for making it The composite article comprises [58] Field gstainless steel bonded to a non-ferrous metal that is felectrochemically anodic to carbon steel. Formation of the bond which ispreferably mechanical is depen- [56] Rderences (med dent upon heat andpressure. The bond is preferably Nl T E PATENTS produced by usingnon-ferrous metal in wire form and without any mechanical or chemicalcleaning of the 269L815 10/1954 Boessenkool ct stainless steel andnon-ferrous metal for removal of 2,753,623 7/1956 Boessenkool et al....29/498 X Surface oxides 3,025,225 3/1962 Snyder eta1...............204/l45 3,269,004 8/ 1966 Smith Jr. et a1 ..29/498 X 13Claims, 2 Drawing Figures FIGJ PATENTEDAUGZZ a 3.685136 SHEET 1 If? 2FIGJ WILLIAM 0. H58 VNL'Z, J2.

a rrazhlgx METHOD OF MAKING A PRESSURE BONDED COMPOSITE MEMBER Thisapplication is a continuation-in-part of copending application Ser. No.706,495 filed Feb. l9, 1968 now abandoned.

The invention relates to a method for producing a bond between stainlesssteel and a dissimilar metal which is electrochemically anodic to carbonsteel and to the product produced thereby and more particularly to amethod for forming a sacrificial metal coated stainless steel stripsuitable for fabrication into an automotive trim member and to theproduct produced thereby.

Stainless steel is known to serve a particularly good purpose in thoseapplications where brightness and stain resistance to a variety ofcontaminants is required and is ideally suited for use as automotivetrim. However, since most automotive bodies are constructed of carbonsteel, the use of stainless steel can result in galvanic corrosion.Carbon steel is anodic to stainless steel and galvanically corrodes inthe vicinity of the stainless steel trim in the presence of anelectrolyte, such as moisture.

To protect carbon steel it has been the practice to coat stainlessstrips with a non-ferrous sacrificial metal which is anodic to mildcarbon steel prior to forming the strips into trim members. At times thecoating was applied to only those sections of the strip which willeventually contact the automotive body, i.e. the return flange of thefabricated trim member. At other times the coating was applied to theentire strip and then subsequent to fabrication removed from allsections except for those which eventually contact the automotive body.Neither of these techniques, however, were found to be completelysatisfactory. This is because prior coating methods, such as vacuumvapor deposition and electroplating, have definite drawbacks. Thesedrawbacks have now been overcome as we have developed a new method forapplying sacrificial metal to stainless steel strips along with theresulting products, improved sacrificial metal coated stainless stripssuitable for fabrication into automotive trim members. The sacrificialmetal of this invention can be aluminum, zinc, cadmium, magnesium, basealloys and mixtures of these metals or any other non-ferrous metal thatis electrochemically anodic to carbon steel.

It is accordingly an object of this invention to provide a process forforming a bond between stainless steel and a dissimilar metal which iselectrochemically anodic to carbon steel.

it is another object of this invention to provide a process for forminga sacrificial metal coated stainless steel strip suitable forfabrication into an automotive trim member.

It is an additional object of this invention to provide a compositearticle comprising stainless steel and a dissimilar metal which iselectrochemically anodic to car bon steel.

It is a further object of this invention to provide a sacrificial metalcoated stainless steel strip suitable for fabrication into an automotivetrim member.

The foregoing and other objects of the invention will be best understoodfrom the following description, reference being had to the accompanyingdrawings, wherein:

FIG. 1 is a photomicrograph at 250x of the cross section of roll bondedaluminum on automotive type 434 stainless steel; and

FIG. 2 is a perspective view of stainless steel strip having aluminumstripes bonded thereto as it appears after rolling.

The bonds of this invention are dependent upon heat and pressure and arepreferably mechanical in nature, i.e. substantially free of diffusion.ln mechanical bonds diffusion is so slight that bond characteristics arenot dependent upon it. This is advantageous as diffusion dependentmetallurgical bonds are often handicapped by the formation of brittleinter-metallic compounds. Accordingly, the stainless steel member isheated to a temperature of at least 375' F., preferably 375"575 F., andthe sacrificial metal, preferably aluminum, is heated to a temperatureof at least 140 F., preferably l40575 F. The members are then pressedtogether, preferably roll pressed, with sufficient force to join them. i

The preferred temperature ranges of 375-575 F. for the stainless steeland of l40575 F. for the sacrificial metal were chosen since they assurethe formation of the preferred substantially diffusionless mechanicalbonds. Diffusion rates increase very rapidly with temperature, and atsome bonding temperature above 575 F. the bond is no longersubstantially diffusionless. Furthermore, use of temperatures below 575F. imparts a substantial advantage to the process of this invention asthey enable bonding to occur in the absence of a protectivenon-oxidizing atmosphere which is required at higher temperatures. Thenon-oxidizing atmosphere is needed at higher temperatures, i.e.temperatures in excess of 575 F., since the stainless steel heat tintsin the absence of it.

Heating of the members can be accomplished by any suitable well knownheating means, e. g. infra red, lasers, and self-resistance. Since themass of sacrificial metal is relatively small in comparison to thestainless steel, it has been found preferable to subject only thestainless steel to the heating means and to supply the sacrificial metalwith sufficient heat by bringing it into contact with the stainlessprior to the application of bonding pressure.

Application of pressure can be by any suitable well known means such asplatens and rolls. Roll pressing is, however, preferred. Pressure shouldbe sufficient to cause a reduction in the sacrificial metal of at least50 percent and preferably percent. Reduction in the stainless steelshould preferably be no greater than 15 percent. Greater reductionselongate the stainless to a degree which imparts dullness, therebydestroying its mirror finish and also cause localized variation inmechanical properties. it is sometimes desirable to soften thesacrificial metal prior to bonding, through annealing. For example, itis preferable that aluminum be in the dead soft fully annealedcondition.

The sacrificial metal can be in the form of wire, sheet, foil or powder.Experimentation has shown wire to be in the preferred form. Wire permitsa greater reduction in the sacrificial metal and produces an "explosivetype effect" during bonding. More specifically, it results in theextrusion of sacrificial metal perpendicular to the rolling direction aswell as flattening in the rolling direction. Furthermore, the greaterreduction causes fresh sacrificial metal, i.e. metal not exposed to theenvironments, to contact the stainless at the bonding interface.

To insure a continuous operation when roll bonding wire it has beenfound preferable, but not necessary, to take certain measures whichprevent the roll from seizing the wire. These measures include theapplication of wet and dry lubricants and release agents to the rolls."Wet lubricants, however, should be used in minimal amounts as theirpresence at the bonding in terface could interfere with the formation ofsatisfactory bonds. An alternative measure utilizes a textured rollwhich is adequately cooled to maintain a temperature differentialbetween it and the strip, e. g. 200 F.

A further feature of the process of this invention is that the membersbeing bonded do not have to be treated, either chemically ormechanically, to remove the normally present surface oxides. This is aconsiderable advantage over prior processes which require a costlyand/or time consuming oxide removal operation. The only surfacerequirement is that the material be degreased or supplied in a degreasedcondition. A preferred surface treatment for bright annealed stainlesssteel is electropassivation, i.e. electrolytic pickling, which removesthe film often formedduring bright annealing. This operation is,however, not a treatment to remove normal surface oxides.Electropassivation is used to produce a surface of solderable qualitywhich happens to improve mechanical bonding. It may be performed ineither bipolar or direct contact systems using suitable electrolytes.

The only limit upon the size of the members being bonded is that imposedby the handling and pressing apparatus. Sacrificial wires, the preferredform of sacrificial metal, generally range in size from 0.0l to 0.025inch. Their size is dependent upon the desired width and thickness ofthe sacrificial metal stripes. The thickness of stripes is preferablybetween 3 and 5 mils and should be at least 0.1 of a mil. A pluralityofwires can be employed to produce stripes wider than those normallyrequired.

Referring now to the drawings, FlG. 1 shows the cross section at 250x ofa composite member produced in accordance with the preferred bondingtemperatures of this invention. It comprises aluminum sacrificial metal1, stainless steel 2 and bonding interface 3. The interface issubstantially free of diffusion between the aluminum and stainless,thereby showing the existence of a mechanical bond. FIG. 2 showsaluminum stripes l4 bonded to the upper planar surface 12 of stainlesssteel strip in a direction substantially parallel to the longitudinaledges 16 of the strip. These stripes were in the fomi of wires prior tobonding and were positioned on the strip by a guiding mechanism. Thenumber of stripes depends upon the desired width of the stainless trimmembers, the desired width of the sacrificial stripes and the width ofthe stainless strips. After bonding, strips 10 are longitudinally slitalong the longitudinal axes of stripes 14, thereby producing strips of adesired width with sacrificial stripes on each edge, which are suitablefor fabrication into automobile trim members.

The following example is illustrative of an embodiment of thisinvention. Type 434 stainless steel strip 0.0l 8 inch thick and about 3inches wide was heated to approximately 500 F. Four aluminum wires 0.0 l8 inch in diameter were guided into position on the stainless strip andthereby heated by contact with the heated strip to a temperatureapproaching 500 F. The positioned wires and stainless strip were thenpassed through a rolling mill wherein the aluminum wires becamemechanically bonded to the stainless steel forming stripes ofsacrificial metal thereon. These stripes were approximately 4 milsthick. The reduction in the stainless steel was not appreciable. Afterbonding, the sacrificial metal coated stainless strip was slit into twomultiples L325 inches wide so as to locate the stripes at the edges andthen successfully fabricated into trim members.

It will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in connection with specificexamples thereof will suggest various other modifications andapplications of the same. It is accordingly desired that in construingthe breadth of the appended claims they shall not be limited to thespecific examples of the invention described herein.

We claim: 7

l. A method of forming a substantially diffusionless mechanical pressurebond between stainless steel and non-ferrous metallic wire from thegroup consisting of aluminum, zinc, cadmium, magnesium and base alloysand mixtures thereof comprising the following steps:

a. heating said stainless steel to a temperature of from about 375 to575 F;

b. heating said non-ferrous metallic wire to a temperature of from aboutl40 to 575 F; and

c. pressing said heated stainless steel and said heated non-ferrousmetallic wire together with sufficient pressure to flatten said wire andeffect a substantially diffusionless mechanical pressure bondtherebetween.

2. A method according to claim 1 wherein said nonferrous metallic wireis aluminum.

3. A method according to claim 1 wherein said pressure is applied byrolling.

4. A method according to claim 1 wherein said nonferrous metallic wireis heated by said stainless steel.

5. A method according to claim 1 wherein said nonferrous metallic wireis annealed prior to said heating of said non-ferrous metallic wire.

6. A method according to claim 1 wherein said pressing produces areduction in the non-ferrous metallic wire in excess of about 50percent.

7. A method according to claim 1 wherein said stainless steel iselectropassivated prior to said heating of said stainless steel.

8. A method according to claim 1 wherein said heat and pressure areapplied in a non-protective atmosphere.

9. A method according to claim I wherein said nonferrous metallic wireis bonded without removing surface oxides.

10. A method according to claim 1 wherein a plurality of non-ferrousmetallic wires are mechanically bonded to said stainless steel.

11. A method according to claim 10 wherein said wires range from 0.015to 0.025 inches in diameter.

12. A method according to claim 11 wherein said bonded and compressedwires range from 0.003 to 0.005 inches in thickness.

13. A method according to claim 1 wherein said pressure is applied byrolling said stainless steel against said non-ferrous metallic wire andincluding the step of applying a release agent to the roll whichcontacts said wire.

2. A method according to claim 1 wherein said non-ferrous metallic wireis aluminum.
 3. A method according to claim 1 wherein said pressure isapplied by rolling.
 4. A method according to claim 1 wherein saidnon-ferrous metallic wire is heated by said stainless steel.
 5. A methodaccording to claim 1 wherein said non-ferrous metallic wire is annealedprior to said heating of said non-ferrous metallic wire.
 6. A methodaccording to claim 1 wherein said pressing produces a reduction in thenon-ferrous metallic wire in excess of about 50 percent.
 7. A methodaccording to claim 1 wherein said stainless steel is electropassivatedprior to said heating of said stainless steel.
 8. A method according toclaim 1 wherein said heat and pressure are applied in a non-protectiveatmosphere.
 9. A method according to claim 1 wherein said non-ferrousmetallic wire is bonded without removing surface oxides.
 10. A methodaccording to claim 1 wherein a plurality of non-ferrous metallic wiresare mechanically bonded to said stainless steel.
 11. A method accordingto claim 10 wherein said wires range from 0.015 to 0.025 inches indiameter.
 12. A method according to claim 11 wherein said bonded andcompressed wires range from 0.003 to 0.005 inches in thickness.
 13. Amethod according to claim 1 wherein said pressure is applied by rollingsaid stainless steel against said non-ferrous metallic wire andincluding the step of applying a release agent to the roll whichcontacts said wire.